How to Make It
Stir together water, greens, cola, ham hock, onion, garlic, kosher salt, crushed red pepper, and 2 teaspoons of the black pepper in a large pot. Bring to a boil over medium-high; reduce heat to medium-low, and simmer until greens and ham hock meat are tender, about 2 hours. (Mustard greens will take 45 minutes.)
Sprinkle hanger steak with sea salt and remaining 1 teaspoon black pepper, and rub into steak. Let stand 10 minutes.
Heat a well-seasoned 12-inch cast-iron skillet over medium-high. Add steak; cook, turning occasionally, 10 to 12 minutes for medium-rare or until desired degree of doneness. Remove steak from skillet, and let rest until just cool enough to handle but still very warm, about 7 minutes.
While hanger steak rests, thinly slice or shave grapes, removing seeds as you go.
Using a slotted spoon, divide greens evenly among 8 serving plates. Slice steak, and place on top of greens. Top with sliced grapes, and serve immediately.